Previous versions of this paperwere presented at the 2003AnnualMeeting of theMidwest Political ScienceAssociation, Chicago, Illinois and the 2005AnnualMeeting of the SouthernPolitical Science Association, New Orleans, Louisiana; I thank Kyle Saunders and Patrick Cotter for their helpful comments at those conferences. I also thank the participants in the Duke University PARISS Seminar (in partic...
